Airborne ultrafine particles (UFPs) are particles with a diameter in the range approximately from 10 to 500 nm. In the technical literature, the term UFP is sometimes also used for particles with a diameter below 300 nm. Because inhalation of UFPs is known to be harmful to human health, UFP sensors may be used for monitoring the quality of air, in particular the UFP pollution level in the air, both in the indoor and outdoor environment. Appropriate measures to reduce exposure to airborne UFPs can then be taken when deemed necessary on the basis of the UFP sensor signals. For instance, an air processing system arranged to remove airborne UFPs from the indoor environment can be run more economically when at least the actual indoor UFP concentration level is known. Apart from UFPs smaller than about 500 nm, also airborne fine particles (FPs) sized between about 300 nm and 10 μm are of interest. Even though FPs as a class of particles are believed to be less hazardous than UFPs, any airborne particle smaller than 10 μm can potentially create hazard because it is inhalable and capable of reaching and depositing in the deep alveolar region of the lungs. Thus, also the measurement of airborne FPs along with the measurement of airborne UFPs remains a worthwhile pursuit.
Indoor measurements are preferably performed in premises wherein people live or work, or wherein UFPs are produced, such as cooking areas. To interfere as little as possible with normal human activities, UFP sensors should be small, unobtrusive, and also noiseless. Since the UFP concentration may vary significantly between different rooms, several measurement points are often required within a single residence, and it is therefore desirable to keep the cost per sensor low.
Co-pending patent application WO 2007/000710 relates to a UFP sensor device in which electric precipitation is used to assess the size and concentration of airborne UFPs. A high-voltage discharge electrode is used to generate and emit airborne ions into an airflow entering the device. Part of the airborne ions attach to the UFPs in the airflow, thereby charging them. The charged particles are subsequently captured by a mechanical filter that is disposed in an earthed conducting Faraday cage. The concentration of charged particles can be evaluated by measuring the amount of particle-bound electric charge that deposits inside the mechanical filter. Before reaching the filter, the airflow passes through a parallel-plate precipitation section wherein either or not an electrostatic field can be provided to remove either or not, respectively, part of the charged particles in a certain size range from the airflow by means of electrostatic precipitation. This enables the generation of two different measurement signals, one signal being associated with the measurement of all charged particles in the airflow, the other signal being associated with the measurement of the remaining charged particles in the airflow after part of the charged particles has been removed from the airflow by means of electrostatic precipitation. Combination of the two signals allows both the particle number concentration (as used herein, the particle number concentration is the number of airborne particles in a unit volume of air) and the number-averaged particle diameter to be inferred.
The UFP sensor disclosed in WO 2007/000710 is advantageous for its robust construction wherein the magnitude of the airflow through the sensor is primarily determined by the characteristics of the pump or ventilator drawing air through the sensor and by the pressure drop incurred by the mechanical filter inside the Faraday cage. Small environmental air pressure differences between the air entry and air exit of the sensor do not substantially affect the airflow through the sensor.
There are also known sensors in which an airflow is created by the thermal chimney effect induced by heat that is continuously supplied to air at the bottom end of an open air passage inside the sensor through which the airflow is passing. The thermal energy needed for inducing such an airflow makes operation of the sensor less economical. Moreover, the thermal chimney effect is only effective when the air passage inside the sensor is positioned vertically, thus limiting the general applicability of the thermal chimney effect.